Biaxially oriented polypropylene metallized films are used for many packaging applications, particularly in food packaging, because they have important sealing and protective qualities. The films minimize the amount of light, moisture, and oxygen which can normally enter an ordinary, unprotected package. The films are often used in food packaging in combination with gas-flushing applications to protect the contents from moisture and oxidation. Also, the films often provide a heat sealable inner layer for bag forming and sealing.
Metallized films used in vertical-form-fill-seal (VFFS) packaging provides an excellent barrier in both unlaminated or laminated forms. However, because of the wide variety of forming collars used, bag sizes, filling speeds, and machine tensions used during the process of bag-forming, the laminated packaging containing the metallized film can be stretched in the packaging machine from 5 to 10% beyond the dimensions of the original film packaging. This stretching may cause fracture or cracks to form in the metal layer of the film. As a result, the packaging loses its protective properties. For instance, oxygen can readily pass through a damaged packaging film and cause unwanted oxidation of the contents.
High barrier metallized OPP films are typically metallized to an optical density range of 2.0-2.4. This has been shown to be adequate to provide good flat sheet (unelongated) barrier properties. However, such an optical density level has not been shown to provide good barrier durability during the bag forming process.
U.S. Pat. No. 5,698,317, the disclosure of which is incorporated herein by reference, discloses the use of a four layer packaging film having a polyolefin resin layer sandwiched between a polyolefin mixed resin layer comprising a petroleum or terpene resin and a heat sealable layer or non-sealable winding layer. A metal layer is then deposited on the surface of the polyolefin mixed resin layer. The metal layer is deposited following the discharge treatment of the polyolefin mixed resin layer.
U.S. Pat. No. 4,297,187, the disclosure of which is incorporated herein by reference, discloses the use of a discharge treatment method on a plastic surface in a controlled atmosphere comprised of N2 and CO2.
The present invention improves upon the moisture and gas barrier properties as well as the durability of the metal layer.